The machines have become sentient and have decided to get rid of all organic life on earth. The surviving humans, gathered on the planet Nova, have come up with a desperate plan. Their last creation, the robot OR-CABE-3 is sent to Earth to steal the blueprints of the machines' central complex. OR-CABE-3 has succeeded in its mission but it must now escape from the enemy underground base and return to the surface, where a shuttle to Nova awaits.

The Last Mission is a side-view arcade game in which the player must explore 94 different screens full of traps and enemies in order to escape. The OR-CABE-3 robot is armed with a laser and can detach its head to visit screens where the rest of its body can't follow. While detached, the robot head expands energy and must soon return to its body. Any hit with an enemy or a trap and the player is sent back to the first screen of the game.